Typically a cryogenic adiabatic demagnetisation refrigerator (ADR) is used for achieving very low temperatures, for example below 1 K, potentially below 1 mK after initial cooling to between 1 K to 4 K using liquid helium or liquid cryogen free cooling techniques. Broadly speaking a magnetocaloric material is employed, typically a paramagnetic material with a high magnetic susceptibility and large entropy change under the application of magnetic fields. This is held at a constant temperature whilst a field is applied and then the field is removed adiabatically allowing the magnetic moments to randomise resulting in a reduction in temperature. The magnetocaloric material is generally a paramagnetic salt such as CMN (cerium magnesium nitrate).
In principle a solid state refrigerator may be based on an electrocaloric or thermoelectric material rather than a magnetocaloric material. The electrocaloric refrigeration method has a similar underlying cooling principle, related to electric rather than magnetic degrees of freedom and alignment of internal electric polarisation. At present, however, suitable electrocaloric materials are less well developed for a working device although alkali halides such as KCl doped with polarisable impurities such as Li+, OH− and CN− show promise.
ADR based technology is employed, inter alia, for space applications (for example J-M Duval et al, ‘A miniature continuous adiabatic de-magnetisation refrigerator with compact shielded super conducting magnets’, in Millimeter and Sub-Millimeter detectors for Astronomy II, Ed J Zmuidzinas et al, SPIE Vol 5498 pp 802-811); further background prior art can be found in Paul A Bromiley, PhD thesis, University of London 1999, ‘Development of an adiabatic de-magnetisation refrigerator for use in space’.
Typically the magnetocaloric material is provided within a cooling ‘pill’, generally a metal cylinder within which crystals of the material are grown. The cylinder may be sealed to inhibit dehydration of the salt. In the pill of Duval et al the salt is grown in a brass can containing copper wires braised to a thermal bus; in that of Bromily's thesis a thermal bus is employed consisting of a set of fins connected to a central pillar. These designs, however, have significant disadvantages. For example the approach of Bromiley involves machining components from solid which is very expensive. In both designs it can take a period of days to weeks to grow the salt crystals, and short pill lifetime and relatively poor thermal conductivity are also problems.
The heat switch in an ADR can also be a source of design difficulties—US2003/0041600 describes an electro-mechanical heat switch but this is complicated, expensive and cumbersome to use in practice.
Other difficulties include sealing the pill to inhibit dehydration, particularly in a vacuum, and under large changes in temperature. This is problematical because epoxy seals tend to crack upon repeated thermal cycling whilst welded seals are expensive and difficult to produce without compromising the performance of the refrigerant through overheating.
There is thus a need for improvements in the design of adiabatic solid-state magnetocaloric and electrocaloric refrigerators.